Method and device for accelerating the decomposition of biogenic organic matter in refuse disposal sites

ABSTRACT

The invention relates to a method and device for accelerating the decomposition of biogenic organic matter in refuse disposal sites. According to the method, at least one suction line is inserted into a disposal site debris heap ( 11 ) via which the arising refuse site gases are removed by suction. According to the invention, the quantity of gas removed by suction in the area of the bottom of the refuse disposal site should be greater than the quantity of gas arising in this area due to decomposition whereby enabling oxygen from the outside air to penetrate into the debris heap and transforming the decomposition process into an, at least to a large extent, aerobic decomposition.

[0001] The invention relates to a method of accelerating thedecomposition of biogenic organics in garbage dumps in which the refuseis layered upon a disposal site ground and is stored in a heap in whicha suction pipe is incorporated by which the gases produced by thegarbage in the heap can be drawn off.

[0002] The invention relates further to a device for carrying out theaforementioned method with at least one suction pipe incorporated in theheap or pile and which extends to the ground region of the dump, atleast one measurement sensor for analyzing and measuring theconcentration of the gases arising in the heap or pile and a controldevice for adjusting the suction power, namely, the through-flowquantity of the drawn off gases in the suction pipe or suction pipes independence upon the gas or gases detected by the measurement sensor andtheir concentration.

[0003] By garbage or refuse here such mixtures will be understood asarise as household waste or household wastes similar to commercial wasteand which aside from nondecomposible inorganic components have organiccomponents which in the presence of oxygen and sufficient moisturedecompose aerobically and in the absence of oxygen decomposeanaerobically. Earlier methods to ensure the greatest possibleprotection of the environment in waste disposal, by garbage separationconditions were provided for aerobic composting or anaerobic sewagetreatment in columns or the like to allow the value of the waste to berecovered without continuously burdening dump sites. Consequentlyaerobically decomposed organic waste can be used as compost. In ananaerobic waste treatment, the recovered methane can be used as valuableenergy. Correspondingly, it is also possible to dry the waste to theextent that it will be sufficiently stable for use as a fuel to recoverthe fuel value.

[0004] Especially in earlier years, the waste that was collected waslayered in refuse dumps or dumped into previously provided or naturallyoccurring burial sites. After filling the capacity of these sites andtheir closure, the dumps or heaps were more or less covered and incertain cases even planted. The decomposition of organic componentsoccurring over time in the covered piles or heaps, however, could reachintolerable concentrations especially in locations close tomunicipalities or within municipalities.

[0005] In such closed dumps, therefore, suction pipes were introducedthrough which the dump gases which were produced could be evacuated. Thestarting high methane concentrations of up to 60 volume percent wereeconomically evacuated and the methane burned in power plants. With timehowever, the decomposition process slows so that the percentage methanegas content in the evacuated dump gases becomes significantly less sothat finally concentrations are reached which exclude economicallycombustion on the one hand but on the other hand still reach values offor example up to 25 volume percent which create the danger ofexplosions in closed spaces. By gas migration, large distances (up to300 meters have been found) can be bridged before the gas emerges forexample in cellars or sewage shafts and there forms dangerous explosivegas mixtures.

[0006] To minimize these dangers, according to the state of the art, theevacuation of dump gasses is carried out for several hours. For thispurpose, the gas pipes, well points or the like must be distributed overthe entire height of the heap or pile to allow the dump gases to beevacuated and the suction power must be correspondingly matched to thequantities of the dump gases which are produced.

[0007] When the CH₄ concentration drops below about 25 to 27 volumepercent, the combustability of the gas terminates. Either the CH₄composition must be raised above 30 volume percent by means of theadmixture of gas therewith to form a combustible gas mixture or the gasmixture must without the additional mixing of gas therewith be oxidizedin a catalyzer stage to CO₂.

[0008] Based upon the usual configuration of gas wells with sufficientfiltering up to the surface of the heap, a strong suction can beeffected only in the region of the surface and thus there is an aerationof the heap only close to the surface and a conversion of the biogenicorganics in this region. In deeper layers which form, in the greaterpart of the pile there is no change in the conversion conditions.

[0009] Through the biological decomposition on the other hand, as aresult of hydrolysis processes, organic compounds tend to be transformedinto a water soluble form and can be carried away by the percolatingwater. This gives rise to the possibility of ground water loading(measured as biological oxygen demand (BOD) and NH₄ content in thepercolating water. Based upon present understandings, the danger of gasemissions and percolating water emissions can last between 50 and 100years.

[0010] It is therefore the object of the present invention to provide amethod and a device of the type described at the outset which canachieve a minimization of the operating cost in a garbage dump by amassively accelerating decomposition of the biogenic organics in thedump. The duration of quasicomplete decomposition of the biologicallydecomposable organics should be significantly reduced as a rule to lessthan 10 years.

[0011] This object is achieved by the method according to claim 1 aswell as by the device according to claim 5; further developmentsaccording to the invention are described in the dependent claims 2 to 4or 6 to 12.

[0012] The core concept of the present invention resides in that aerobicconditions are created in the heap or pile under which the biologicaldecomposition process is carried out substantially more quickly thananaerobic conditions. By the feature that, in the region of the groundbeneath the dump the gas quantity which is sucked out is greater thanthe gas quantity produced in this region by decomposition and is as afunction of the suction power after some time capable of establishing aforced aerobic condition in the pile.

[0013] Through the cover of the dump, oxygen is sucked with the externalair into the dump. By contrast with the suction devices known from thestate of the art, the suction is effected in the lower portion of thedump, preferably in the lower third, that is preferably in a zone whichis in general up to a height of a maximum of 6 meters above the groundon which the dump rests in a gas-filled region. The suction should inany case be effected close to the base and as far as possible not abovethe bottom region which has here been described. The height of thesuction zone depends however also upon the height of the layered pile orheap. The dump is subjected to oversuction when the suction level in thecentral region of the dump is so established that by means of thesuction applied to the dump, an approximately constant subatmosphericpressure is applied thereto. The dump is so oversuctioned that the flowvelocity in the dump from the suction point to the outer regions of thedump at which the ambient air flows into the latter, falls strongly.Because of the reduced flow velocity in large areas, a local overheatingdoes not occur, there is a formation of shrinkage cracks and otherpassages through which the air preferably flows into the dump.

[0014] Local overheating is the case when, according to the methodsknown from the state of the art, air was forced into the dump throughlances or gas wells. At the entry location in a comparatively smallvolume of the dump, there was the greatest flow with a high oxygenlevel. That resulted in an intensive local biological and chemicalconversion of the biogenic organics. That gave rise in inlet regions tolocal overheating partly leading to local combustion nests. Because ofthe temperature increase the gas which is produced picks up a great dealmore moisture than is introduced by the cold air injected into the dump.The dump is partially dried out, thereby retarding further biologicaltransformation. As a result of the drying out, additional shrinkagecracks arise and the result is an uncontrolled outflow of the injectedair in spite of intensive aeration. Basically it is also possible viathe lances introduced into the heap or pile to blow normal air or oxygenenriched air into the pile. However, such a process is not onlysignificantly more expensive but aside from the aforedescribed problems,is also more expensive from the measurement of process control points ofview.

[0015] According to the invention, the uniform aeration of the entirepile or heap results in an oxygen excess which affects an aerobicconversion whose gaseous end product is CO₂ and H₂O. It is alsoimportant that the evacuation of the gas takes places deep within theheap or pile at the ground supporting the dump and preferably in thelower third of the dump since otherwise the danger arises that theentire quantity of air drawn through the upper layers of the dump pilewill be drawn off and thus pass through the suction slits aftertraversing only a small part of the dump volume, thereby permitting theaerobization and decomposition only in the upper region while the lowerzones to the foot of the dump remain subject to aerobic decompositionwithout change. With the aerobization according to the invention, on theone hand by establishment of aerobic conditions, methane formation isreduced while on the other hand, aerobic transformation to CO₂ isgreatly increased. A high CO₂ concentration with low methane and alsolow oxygen concentrations is a measurable indicator for a goodaerobization if at the starting stages.

[0016] The method according to the invention allows previously practicedfeatures to be combined with methane gas recovery in that when initiallythe suction pipes or well points provided are used to pump out the gasesformed in the pump in the lowest region, preferably the lowest thirdwith the correspondingly high methane content until the methane contentin the gas sinks to a value below which an economical utilization ofmethane, for example by combustion in a thermal power plant is no longerpossible.

[0017] To the extent that measurement sensors (filter cartridges) areprovided at different levels in gas wells (multilevel wells) but outsidethe pipes incorporated in the pump, these filter cartridges can bearranged in a filter cartridge bed. Between these filter cartridge bedsor the filter cartridges, clay blocks are provided to ensure that thedump gas development can be controlled by gas sampling by themeasurement sensors at the different heights within the dump.

[0018] The dump gas development and their compositions indicates whetherthe desired suction power is sufficiently strong to ensure satisfactoryaeration of the dump heap or pile over the entire volume.

[0019] According to a further feature of the invention, the gasevacuation is effected exclusively in the region of the bottom of thepump, preferably in the lower third of the pile or heap and/orpreferably in the region at most six meters above the floor of the dumpin a zone forming a gas-filled region (unsaturated region). At leastabove the previously defined zone there should be no gas evacuationalthough there may be small gas samplings over filter cartridges toenable the gas composition to be explored as a function of the pile orheap height so that the suctioning power may be correspondingly raisedor lowered in the dump region therebelow.

[0020] Preferably the gas evacuation is carried out in the region of thefloor of the dump at a multiplicity of locations which are preferablyequidistant from one another. In this manner, one takes intoconsideration the fact that the dump extends over a large area so that acorresponding number of suction pipes or wells are advantageous.

[0021] As has already been indicated by example, a so-called multilevelwell can be used at which the gas composition at different heights inthe pile or heap can be measured at different locations, themeasurements evaluated and the suction power so controlled as a functionthereof so that at each location in the pile or heap an oxygen contentis established as will be necessary for an aerobic decomposition. If oneuses multiple wells or separate suction pipes, the throughflow volumescan be individually varied in each suction pipe or in each well for thedump gas which is sucked out in order to aerate more strongly regionswhich might still be undergoing aerobic decomposition and to optimizethe aerobic decomposition.

[0022] The described method is carried out by the apparatus according toclaim 5 which, according to the invention, is characterized in that thesuction opening of the suction pipe or pipes are located in the regionof the floor of the dump, preferably in a region which is in the lowerthird of the pile or heap or in a region which is at most 6 meters fromthe floor of the floor of the dump. Preferably a plurality, especiallymutually equidistant suction pipes are distributed in the heap or pile.In accordance with a further feature of the invention a multiplicity ofsuction pipes are so installed that their positions, spacings andsuction power are so selected that their effective suction regions atleast bound one another and preferably slightly overlap. Especially thesuction power in each suction pipe should be individually controllable,preferably as a function of the measured gas concentration at theirrespective effective regions. The suction pipes have in their upperregions (in which no dump gases should be evacuated) a gas-tight shellas well as a clay block lying outwardly to prevent gas short circuitsparallel to the unslit regions of the suction pipe.

[0023] The present invention is described in detail in connection withan example. The drawing shows

[0024]FIG. 1 a cross section through a buried dump and

[0025]FIG. 2 a partial cross section through a multilevel well.

[0026] According to FIG. 1, in a buried dump 10 containing refuseforming a pile or heap (11) in layers and covered at the top by anatural covering (12) for example in the form of planted soil, in theillustrated embodiment two bores (13) and (14) are provided in whichrespective tubes 16 with closed walls are introduced. In the presentcase, in addition, two filter tubes (17) are provided around which agas-permeable gravel bed (18) is arranged. Thereover or between them areearthenware blocks (19) which prevent the gas from passing along thesolid pipe or through the filter gravel layers and being drawn into thesuction wells and thereby reduce the effectivity of the suction wells orsuction regions. In the illustrated case, only through the gravelfilters and the corresponding solid wall pipes (16) and (17) can the gasbe pumped out by a motor-driven pump (20). In order to ensure that thegas is drawn out close to the base of the dump, only lower filters belowthe clay pipes are effective to draw the gas out of the system. This canalso be realized by including a liner for the upper filter stretches.

[0027] According to a further configuration of the invention, the linercan also be height adjustable (FIG. 1). This has the advantage that anupper filter (18) can be shut off so that the suction develops furtherin the depth of the heap or pile.

[0028] Finally, the well can extend downwardly until below the groundwater level so that via corresponding additional suction pipes orpressure pumps, ground water samples can be withdrawn.

[0029] Both wells (13) and (14) extend to a point below the floor of thedump and can terminate below the ground water level so that byappropriate pumping, water samples can be removed.

[0030] Preferably over the entire area of the dump which can extend fora number of hectares, wells (13), (14) or suction pipes can be uniformlydistributed and connected with a single suction apparatus or with amultiplicity of suction apparatuses. The suction power of the entireapparatus can be controlled by suction motors operated by controllablefrequency converters.

[0031] Through individual regulation of the suction power or valvesprovided in each suction pipe, the gas flow can be so controlled that inthe pile or heap (11) a uniform suction pressure is achieved. In thismanner the suction power can be so adjusted that more gas is withdrawnthan is produced by the decomposition processes. Hitherto, suctionpowers were produced which were higher by a factor of about 30 than thegas formation rate measured prior to the beginning of aerobization andunder anaerobic conditions. As a consequence of the air permeability ofthe covering (12), air flows from the surrounding into the dump and hasa component thereof oxygen which passes into the heap or pile (11). Thisair must forcibly traverse the dump material and for that reason, apartfrom small amounts drawn off as samples, gas suction is locally limitedto the region lying above the floor (21) of the dump, for example in thelower third of the height of the layer than this floor. By flowthroughput control in each suction pipe a controlled gas exchange can beobtained over large parts of the body of the dump.

[0032] From gas samples which were taken through the filter cartridges(22) preferably in each of the wells provided or from the head of thewell, gas concentrations are determined from the gas sucked up orpreferably from separate multilevel wells (FIG. 2) thereby determiningthe gas composition and the quantity of the dump gases which arise perunit of time in terms of the spatial distribution so that any deviationsfrom a desired optimal state can locally be evaluated and the quantityof the dump gas sucked out can be locally increased or reducedaccordingly. The measurement sensors (22) or the filter cartridges thussupply the respective actual value signals for pump powers which are tobe employed.

[0033] In order to optionally shift the effective suction region furtherdownwardly, optionally a height adjustable, solid shell tube can bearranged in a suction well in a shiftable manner so that the gaswithdrawal in the region of the upper gravel packing (18) can be closedoff and the suction limited to the lower gravel packing (18). Whethersuch a shiftable tube is provided or at what height above the floor (21)of the dump the substantially single gas evacuation is to be carried outdepends substantially upon the gas permeability of the heap and themaximum possible suction power as well as the type of decomposition,which can be still anaerobic or can be aerobic. To accelerate thedecomposition of the organics in the heap (11), in any case a maximumpossible aerobic composition is desirable since the decomposition underthese conditions is many times more rapid than with aerobicdecomposition.

[0034] According to a further aspect of the invention, gas compositionas well as the subatmospheric pressure developed in the body of therefuse dump is detected at different levels by separate multilevel wells(FIG. 2). In a bore which has not been provided with a pipe lining,layerwise, preferably respectively in one meter layers, gravel (18) andclay binder (19) are provided. In the gravel layers, during theformation thereof respective filter cartridges are introduced and areconnected by measuring gas pipes to the surface. Because of the claylayers, the individual gravel layers are pneumatically separated fromone another.

[0035] Via the individual measuring gas hoses, gas can be withdrawn fromthe individual discrete layers for analysis. Additionally thesubatmospheric pressure in the individual layers can be determined. In aconcrete example, the gas formation rate under anaerobic conditions wereabout 3 m³ per hour. The methane concentration amounted about to 30volume percent. The gas was recovered with a gas suction plant with 28wells.

[0036] Over two newly provided gas wells with suction close to the baseof the dump, initially 100 m³ per hour was withdrawn. Thereafter thesuction power was increased until about 500 m³ per hour of gas waswithdrawn. The starting CH₄ concentration in the withdrawn gas amountedinitially to about 50 volume percent. By increasing the suction power to400 to 500 m³/h, the methane concentration in the gas withdrawn from thedump falls continuously to below 5 volume percent. In the same time theCO₂ content in the evacuated dump gas falls from about 25 volume percentonly to 22 volume percent. Oxygen was not detectable. The total oxygenwas aerobically converted to the CO₂. The reduction in the methanecontent depended upon the oversupply of the regions traversed by thedrawn-in gas with atmospheric oxygen and the transformation from ananaerobic state to an aerobic and thus as reduction in methane formationand in addition to a dilution effect. The CO₂ concentration dependedlargely upon the conversion of biogenic organics with the atmosphericoxygen which was introduced.

[0037] The target of this aerobization phenomenon is the transformationfrom the anaerobic conversion of the organics with methane formationinto a much more rapid and intensive aerobic transformation with carbondioxide production. Critical emission paths are suppressed in that onthe one hand explosion dangers with methane are reduced and on the otherhand a rapid aerobic decomposition greatly reduces the carbon containingmaterials which are soluble in percolating water, measured as biologicaloxygen demand.

[0038] The described features are appropriate to buried dumps as well asfor surface dumps.

[0039] The present invention also encompasses such fields of use inwhich, especially with dumps of considerable height, the dump has layersbetween layers of refuse of a binder material such as clay, loam or likesubstances in layers between the refuse and subdividing the dump into aplurality of horizontal regions. In such cases, each horizontal regionmust be treated as a separate dump. Preliminary tests have shown thatgas evacuation can be carried out in each of the individual horizonallayers together with specific gas concentration measurements such thatthe gas evacuation can then be carried out in correspondence with themethod according to the invention.

1. A method of accelerating the decomposition of biogenic organics ingarbage dumps in which the garbage is deposited upon a garbage dumpground (21) in a heap or pile (11) in which a suction line is includedby which the dump gases produced the heap or pile are sucked out,characterized in, that the gas quantity sucked from the region of thedump ground is greater than the gas quantity arising in this region fromdecomposition so that oxygen in atmospheric air from the exteriorpenetrates into the pile or heap (11) and transforms the decompositionprocess into a decomposition which is at least mostly an aerobicdecomposition.
 2. The method according to claim 1, characterized in thatthe sucking out of the gas is conducted exclusively in the region of thedump ground, preferably in the lower third of the pile or heap (11)and/or in a zone which extends to 6 meters above the dump ground (21).3. The method according to claim 1 or 2, characterized in that thesuctioning out of the gas is carried out at a multiplicity of locationsarranged in the region of the dump ground and which are preferablyequidistant from one another.
 4. The method according to one of claim 1to 3, characterized in that the suction power is so controlled independence upon the gas composition measured at different heights in thepile or heap that each location in the pile or heap an oxygen content isestablished as is required for an aerobic decomposition.
 5. Theapparatus for carrying out the method according to one of claim 1 to 4,with at least one suction pipe included in the pile or heap andextending to the ground region of the dump, at least one measurementsensor for analyzing and measuring the concentration of the gasesarising in the pile or heap and a control device for adjusting the pumppower, namely the through-flow quantity of the gas drawn off in thesuction pipe or the suction pipes as a function of the gases detected bythe measurement sensor and their concentration characterized in that thesuction opening of the suction pipe or the suction openings of thesuction pipes are arranged in the region of the ground of the dump,preferably in the lower third of the dump pile or heap and/or in aregion which extends through a maximum of 6 meters above the dump orheap floor (2 e) from the dump or heap floor (21).
 6. The apparatusaccording to claim 5, characterized in that a plurality preferablymutually equidistantly arranged suction devices are incorporated in theheap or pile (11).
 7. The apparatus according to claim 5 or 6,characterized in that a plurality of suction pipes are installed whosepositions, spacing and pump capacity are so selected that theirrespective suction effective regions at least are adjustment andpreferably overlap.
 8. The apparatus according to claim 7, characterizedin that the pump power in each suction pipe is individuallycontrollable, preferably in dependence upon the gas concentrationsmeasured in the respective effective region.
 9. The apparatus accordingto one of claims 5 to 8, characterized in that the suction power (filterstretches) in their upper regions are limited by a gas-tight shell,optionally by a liner incorporated therein and only in their lowerregions have one or more gas suction openings.
 10. The apparatusaccording to one of the claims 5 to 7, characterized in that the suctionlevel extends to below the ground water level.
 11. An apparatus formeasuring the gas position at different depths, characterized in thatfilter cartridges for gas sample-taking are embedded in a bore inpreferably one meter great gravel layers in which preferably each onemeter thick layer of filter gravel and clay are disposed in layersalternately with one another.